This invention relates generally to focused ion beam (FIB) systems and methods, and more particularly to a low temperature bright microplasma ion source for FIB applications.
Focused ion beam processes are widely used in the semiconductor industry for applications such as integrated circuit (IC) circuit editing (CE) and for debugging and verification of the functionality of ICs. Circuit editing involves modification of individual IC circuits in order to correct design or manufacturing errors that cause IC malfunctions. Currently, FIB systems use a finely focused beam of gallium ions that can be operated in a range of beam currents from several pico-Amperes (pA) up to tens of nano-Amperes (nA). Gallium ion beam technology uses a liquid metal ion source (LMIS) that enables a Ga+ ion beam to be focused down to a nanometer size at low beam currents (on the order of 1-10 pA), which is important and required for successful CE operations on ICs manufactured using current technologies.
Although current LMIS technology has the ability to focus a Ga+ ion beam down to nanometer scale, the use of Ga+ ions is becoming problematic for CE applications on more advanced ICs. This is happening for two main reasons. First, as IC technology is moving towards increasing complexity, the dimensions of all of the IC components scale down. This includes the vertical (depth) dimensions between metallization layers and the thicknesses of the layers themselves, including the “active silicon” layer. Currently 30 keV Ga+ ions are used in FIB technology, a relatively high energy that is needed to focus the beam down to a nanometer sized spot. But at this energy, Ga+ ions are implanted into a modern IC structure to a depth which is comparable to the thicknesses of the metal and active layers. This Ga+ ion implantation and the accompanying atomic cascades that result from the implantation combine to create damaged layers in the IC structure that are comparable in their dimensions to the functional layers of the IC. This damage can compromise the ability of the IC to work properly after such a modification. Secondly, since Ga is a conductive metal, the implantation of Ga+ ions can cause electrical short circuiting of the IC layers to each other or other malfunctioning of the IC.
To minimize such problems, it is desirable to limit the depth of penetration of the ions into the IC materials, which may be accomplished, for instance, by using heavier ions that have shallower penetration ranges into the IC materials. Additionally, it is desirable to use ions that, when embedded in the IC substrate material, do not form an electrically conductive layer. For example, non-conductive ions, as of inert gases, may be used in a FIB to avoid the electrical issues associated with Ga+ ions since they do not form a conducting layer or conducting inclusions. Xenon (Xe) is an inert gas which has a mass (132 amu) that is about twice that of Ga (69 amu). Xenon produces Xe+ and Xe2+ ions which can be obtained from a plasma. However, previous attempts to use a plasma source for forming a focused Xe+ or Xe2+ ion beam have not been successful because of the inability to obtain an ion beam with a suitable current density that can be focused down to the required nanometer scale spot size, which is required for processing modern ICs. The reason for the inability to finely focus the ions from this conventional plasma source lies in the well-established physics of glow discharges, including evolutions of plasma discharge regimes with growing electric current and gas pressure. For a FIB application, an ion source should be “bright”, i.e., have an ion current density of at least about 1 A/cm2 or higher. Otherwise, either the FIB operation would proceed too slowly or the ion source size would have to be so large that fine focusing would not be possible.
Conventional (macroscopic-sized) plasma sources have a plasma volume of the order of 1 cm3 and an electrode area of about 1 cm2. In a conventional plasma source having a discharge volume on the order of 1 cm3 with electrode surface area of about 1 cm2, an ion current density of 1 A/cm2 would require a current of approximately 1 A which is very high. At this current, the plasma would become unstable and unusable. Furthermore, in order to focus the ions down to a nanometer size for CE operations, it is important that the mean ion energy (which is proportional to the ion temperature) not exceed a few eV. Otherwise, the ion energy spread would create chromatic aberrations that would excessively broaden the spot size of the focused beam and make it unusable for CE on newer ICs.
The two conditions, i.e., high current density (brightness) and low temperature (coldness) of the ions, have to be obtained simultaneously, which cannot be done in a conventional normal glow discharge plasma. With a normal glow discharge (NGD), although the temperature of the gas including the ions can be quite low (approximately 300-1000 K) in the positive column, the electrons in the discharge have an effective temperature typically of about 1-3 eV (approximately 12,000-36,000 K). The ion temperature in the plasma cathode sheath from which ions would be extracted for the use in a FIB is typically of the order of 0.5-1 eV, whereas the electron temperature (defined in terms of the mean electron energy) in the cathode sheath can reach approximately 10 eV. This state with widely separated electron and gas/ion temperatures can be sustained by a relatively low rate of Joule heat release and a sufficient rate of the gas natural cooling.
The current density J in normal glow discharges is proportional to the square of the gas pressure, p, and the ratio J/p2 is on the order of 10 μA/(cm2×Torr2) for Xe. Therefore, in order to obtain a current density of about 1 A/cm2, a normal glow discharge in Xe would have to operate at a very high pressure of about 300 Torr or greater, which is impractical. In conventional macroscopic (i.e., geometric sizes approximately 1 cm) plasmas, normal glow discharges cannot be sustained at such high pressures and high current densities. Increasing gas pressure and current density eventually leads to the onset of abnormal glow discharge (AGD) which is characterized by elevated currents, but also a significant increase in the cathode voltage drop, resulting in a very high temperature of the ions in the cathode sheath, about 10 eV and higher at current densities of the order of 1 A/cm2. Moreover, the AGD regime at high currents is susceptible to instabilities and, in particular, to constriction and arcing (glow-to-arc transition). The onset of constriction and arcing instabilities limits the current in glow discharges in inert gases typically to about 10−2 A. Accordingly, current densities in conventional plasmas are limited to about 10−2 A/cm2 which are far lower than required.
It is desirable to provide bright low temperature plasma sources of non-conductive high mass ions, as for example inert ions of inert gases such as xenon, for FIB operations that avoid the foregoing and other problems of conventional plasma sources, and it is to these ends that the present invention is directed.